INTRODUCTION

The cervical vertebra column supporting the head comprises seven vertebrae. The first vertebra (C1) or atlas and the second vertebra or axis together form the superior or suboccipital segment connecting the spine to the occiput and involving a complex chain of joints. Suboccipital muscles attached to this region determine head posture, controlling fine through complicated movements for compound flexion and extension, as well as lateral flexion with rotation.1

Dimensions of C1 as well as head and neck posture are associated with factors such as craniofacial morphology, including the cranial base,2–4 upper airway space,5 occlusion,2–7 and temporomandibular disorders.8,9 Furthermore, head posture is linked to the development and function of dentofacial structures.9 The relationship between C1 dimensions and craniocervical posture has also been studied.2 Cervical posture was linked to mandibular length, with longer mandibles associated with cervical columns more inclined to the true horizontal.10 Mandibular length was also directly correlated with straightness of the cervical column (ie, a lower cervical lordosis angle).11 However, no previous studies have described the relationship between cervical vertebra morphology using three-dimensional imaging and maxillofacial morphology.

Computed tomography (CT) was first considered for such a study because of the accurate three-dimensional imaging that is possible with CT. However, limitations such as radiation, machine size, and cost made this approach impractical. The more recently established cone-beam computed tomography (CBCT) presents a more feasible alternative, as these lower-cost and smaller machines still produce high-quality data. With several CBCT scanners now available, involving lower radiation dosages12–15 and lower costs,16 three-dimensional (3D) radiography is becoming more commonplace in the dental profession as a valuable diagnostic tool.

This study investigated the detailed morphology of cervical vertebrae in three dimensions in subjects with different anteroposterior skeletal patterning.

MATERIALS AND METHODS

Measurements

CBCT images were used to assess the maxillofacial characteristics of Class II and Class III subjects, SNA, SNB, ANB, and mandibular plane (linear measurements detailed in Tables 1 and 2). Figure 4 demonstrates both the angular and linear measurements obtained by CBCT. Of those obtained, eight linear measurements and one angular measurement were used to assess cervical vertebra morphology with different sagittal skeletal patterns.

Table 1 The Maxillofacial Characteristics of Class II and Class III Groups

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Table 2 Mean Value of Cervical Vertebra Measurements

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1. Horizontal outer anteroposterior (AP) diameter of the first cervical vertebra (C1)17 (mm) (HOAPC1; Figure 1)

2. Horizontal inner AP diameter of C117 (mm) (HIAPC1; Figure 1)

3. Horizontal outer transverse diameter of C117 (mm) (HOTDC1; Figure 1)

4. Distance between outer margin of transverse foramen and outer margin of lateral mass17 (mm) (outer margin; Figure 1)

5. AP diameter of superior surface of C1 anterior arch17 (mm) (superior surface; Figure 1)

6. Lateral outer AP diameter of C118 (mm) (LOAPC1; Figure 2)

7. Height of the atlas dorsal arch18 (mm) (dorsal arch; Figure 2)

8. Frontal outer transverse diameter of C1 (mm) (FOTDC117; Figure 3)

9. Angle along axis line of the dens to occlusal plane (dens angle19 [degrees]; Figure 4)

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RESULTS

Table 1 details the mean value of maxillofacial dimensions in the Class II and Class III groups. No statistically significant difference in SNA and mandibular plane angle was noted between Class II and Class III subjects.

Table 2 details the mean value of cervical vertebra measurements in Class II and Class III groups. Only the height of the atlas dorsal arch showed a significant difference between Class II and Class III (P  =  .029), with the other measurements not significantly different among subjects regardless of skeletal class.

DISCUSSION

Skeletal class and cervico-vertebral anatomy both are associated with craniofacial structure. CT imaging has proved remarkably accurate in these association studies for linear,20–23 geometric,24 and volumetric25,26 measurements within the maxillofacial complex. Further to this, we now reveal differences in cervical vertebra morphology in subjects with different anteroposterior jaw relationships.

This study demonstrated significant differences between Class II and Class III subjects in height of the atlas dorsal arch. A pattern of positive correlation between mandibular length (Ar-Me) and the anteroposterior length of the atlas was previously shown to increase in growing subjects.18 In addition, morphological analysis of the atlas dorsal arch revealed a clear association with the growth direction of the mandible, whereby a lower atlas dorsal arch indicated less horizontal growth of the mandible.27 This is probably due to the fact that subjects with a low dorsal arch had a relatively elevated head position and thus altered suprahyoidal muscular activity, which would permanently affect the position of the mandible.27 Therefore, it was proposed that understanding the form of the cervical vertebra and the orthodontic treatment would enhance the treatment plan for subsequent growth.2

This study found no difference in mandibular plane angle between Class II and Class III groups (Table 1). Therefore, the variation demonstrated here in height of the atlas dorsal arch seems independent of craniofacial vertical differences. Several studies have suggested that craniofacial vertical differences are related to differences in cervical morphology.6,7 The present findings showed no difference in outer margin values between groups, although the vertical differences in craniofacial morphology were related to the outer margin measurements.28

The anatomy and position of the cervical curvature in space depend on various body factors, including ethnicity,29–31 gender,29,30,32–34 age,3,32,33 stature,35 and craniofacial morphology.4,10,11,36–39 Differences in these correlations might be attributed to factors such as racial difference,39 although the association between craniofacial measurements and atlas morphology varied with sex and age. The growth directions in cervical vertebrae and mandible growth are certainly associated, although influences here of gender and racial differences are possible.18 Moreover, this study suggested that Japanese female Class II subjects had lower dorsal arch heights than Class III subjects.

CONCLUSIONS

• This study confirmed past findings that Class II subjects are significantly shorter in terms of atlas dorsal arch height. Our findings on the relationship of anteroposterior skeletal pattern to the transverse diameter of C1 parallel the results of previous studies.17,40–43

• The atlas dorsal arch height of cervical vertebrae is affected by the anteroposterior skeletal pattern.